Tônlé Sap (tŏnˈlā säp) [key] [great lake], lake, central Cambodia; largest lake of SE Asia. It occupies the depression of the Cambodian plain and is fed by many streams; the Tônlé Sap River, c.70 mi (110 km) long, drains the lake S into the Mekong River near Phnom Pénh. At low water in the dry season (Nov.–May), the lake covers c.1,100 sq mi (2,850 sq km). During the summer floods, however, the waters of the Mekong back up into Tônlé Sap (which forms a natural reservoir) raising the lake's level c.30 ft (9 m) and more than tripling its area. Approximately 2,500 sq mi (6,475 sq km) of surrounding forest are inundated by the floodwaters and provide a breeding ground for fish. The fisheries of the lake are one of Cambodia's major natural resources. The Tônlé Sap lake and river are also part of an important inland waterway system.